The present review focuses on potential nutritional and physiological functions of betaine in relation to health, performance and carcass characteristics in poultry. Due to its zwitterionic structure, betaine has osmoprotective properties that aid in protecting intestinal cells and thus counteracting performance losses during heat stress and coccidiosis. Hence, betaine might be an appropriate agent in the prevention of heat stress in livestock. In addition, during coccidiosis, the efficacy of betaine appears to be related to the Eimeria species, type of coccidiostat used and the severity of infection. As this compound has methyl donor properties, betaine is involved in methylation reactions in the organism and may therefore partly substitute other methyl group donors such as methionine and choline. Due to its methyl donor and osmoprotectant functions, recent research has shown that betaine improves nutrient digestibility, growth performance and feed conversion in broiler chicks, turkeys and meat ducks. Other potential benefits of the inclusion of betaine in poultry diets are lowering carcass fat content and increasing breast meat yield which may be of interest to satisfy consumer needs.